Press forming process and product formed thereby with high dimensional accuracy, and workpiece used therefor

ABSTRACT

A press forming process involves a stretching press to form a product with a curved portion from a workpiece by imparting a pad portion to the workpiece prior to starting the press forming process. The curved portion is prepared by spreading the pad portion mainly into the longitudinal direction of the workpiece, so that the curved portion is prepared so as to have a spring back of the product minimized.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a press forming process, morespecifically, a press forming process involving stretching press to forma product with a curved portion from a workpiece by which a product withhigh dimensional accuracy can be easily produced.

[0003] 2. Discussion of the Related Art

[0004] In recent years, there is a demand that many sorts of industrialmachines including vehicles should have decreased weight and designedfor safety. As a concrete measure to answer such demand, steel with hightensile strength is employed, which has excellent processingcharacteristics, weldability, and impact resistance in addition to thehigh tensile strength, whereby it has become possible to decrease weightof the product with maintaining the strength thereof.

[0005] However, when steel with high tensile strength is employed as amaterial for obtaining a product therefrom such as a car frame, and ismanufactured by a press forming involving stretching and/or bending, aso-called “spring back” occurs at a curved portion to a larger extent incomparison with commonly employed steel. Thus, it is still difficult toimprove the dimensional accuracy of a product by the press forming.

[0006] The reason why the “spring back” occurs is to be explained withreferring to FIG. 6.

[0007]FIG. 6 shows a frame 1 extended to include a curved portion, andhaving an approximate U-shaped cross-section formed with lateral walls 3at both sides of a top face 2. The curved portions 4 of the lateralwalls 3 are obtained by a stretching press forming operation in order toprepare the frame 1 by the press forming. At this stage, the length ofthe frame I on an outer side of the bending is insufficient to retain aformed shape at the curved portion 4 due to a tensile force.

[0008] The short length at the curved portion 4 causes a spring back ofthe product. In FIG. 6, the normal shape of the frame 1 shown byalternate long and two short dashed lines is deformed at the side underthe tension to the direction shown by arrows X, whereby an undesirableproduct shown by a solid line is obtained. Consequently, the dimensionalaccuracy of the product is decreased.

[0009] A new press forming process is proposed, for instance, asdisclosed in Japanese Kokai Publication 7 (1995)-148527 wherein aproduct is neatly bent to prepare a corner by a part of a workpiecebeing bent once to have a corner with a depression, and then the cornerwith the depression is pressed again so as to have a sharp corner. Inaccordance with this method, the stress distribution on the bent portioncan be well balanced in the plate thickness direction, so that thegeneration of the spring back phenomenon is suppressed.

[0010] Furthermore, Japanese Kokai Publication 11 (1999)-226651 proposesa method wherein the shape of a die is prepared so as to partiallydiffer from the shape of a product to be attained therefrom byestimating deformation of the product, e.g. spring back or warp,followed by performing press forming to give a product, therebymaintaining accuracy of the product.

[0011] In the methods described in the above-mentioned Japanese PatentKokai Publications 7 (1995)-148527 and 11 (1999)-226651, it is necessaryto carry out press forming by the dies having shapes that are largelydifferent from the products to be obtained. The deformation behavior ofthe products, however, does not always follow the estimation, so that itis necessary to repeatedly perform tests for determining the finalshapes of dies, and many manufacturing steps are required also in theactual manufacture.

OBJECTS AND SUMMARY OF THE INVENTION

[0012] It is therefore an object of the present invention to provide apress forming process involving a stretching press to form a productwith a curved portion from a workpiece, wherein a product with highdimensional accuracy can be obtained by an occurrence of a spring backthereof being restrained without using many different dies by performinga step of providing a pad portion to the workpiece prior to starting thepress forming process, the formation of the being completed by spreadingthe pad portion mainly in a longitudinal direction of the workpiece,whereby the curved portion is prepared so as to have a spring back ofthe product minimized.

[0013] Another object of the present invention is to provide theabove-mentioned press forming process involving a stretching press toform a product with a curved portion from a workpiece, by which theabove-mentioned pad portion is easily created, by the pad portion beingin a wave, a bead or a corrugation form in the above-mentioned pressforming.

[0014] A further object of the present invention is to provide theabove-mentioned press forming process involving a stretching press toform a product with a curved portion from a workpiece, by which theoccurrence of a spring back is effectively restrained, by the workpiecebeing made of a steel with a high tensile strength.

[0015] A still further object of the present invention is to provide theabove-mentioned press forming process involving a stretching press toform a product with a curved portion from a workpiece, by which pressforming is smoothly carried out by preventing parts of the workpiecefrom catching or sticking to a die to be used therefor, by the padportion having a shape which does not interfere with the die.

[0016] A still further object of the present invention is to provide theabove-mentioned press forming process involving a stretching press toform a product with a curved portion from a workpiece, by which thedeformation of the product by the spring back is surely restrained by aportion of the workpiece to be stretched being substantially the same asthat of a face of a plate holding member of a die used of the stretchingpress.

[0017] A still further object of the present invention is to provide aproduct with a curved portion, which has high dimensional accuracy evenat the curved portion without causing spring back, prepared by a pressforming process from a workpiece, the press forming process comprisingproviding a pad portion to the workpiece prior to starting the pressforming process, and completing formation of the curved portion byspreading of the pad portion mainly in the longitudinal direction of theworkpiece, whereby the curved portion is prepared so as to have a springback of the product minimized.

[0018] A still further object of the present invention is to provide aworkpiece appropriately used for the above-mentioned press formingprocess to obtain a product with a portion which has high dimensionalaccuracy even at the curved portion without causing spring back, by theprovision of a pad portion at the a portion of the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] A more complete appreciation of the invention and many advantagesthereof will be readily obtained as the same becomes better understoodby reference to the following detailed description when considered inconnection with the accompanying drawings, wherein:

[0020]FIG. 1 is a perspective view of a product obtained by the pressforming process according to the present invention;

[0021]FIG. 2 is a perspective view of a product being pressed by meansof a die by the aid of plate holding members;

[0022]FIG. 3 is a perspective view of a workpiece for explaining anembodiment of a press forming process according to the presentinvention;

[0023]FIG. 4 is a cross-section of a part of the product shown in FIG. 1cut along I-I,

[0024]FIG. 5 is a cross-section of the part shown in FIG. 4 cut alongII; and

[0025]FIG. 6 is a perspective view of a product prepared by a knownpress forming process for explaining a spring back phenomenon.

DETAILED DESCRIPTION OF THE INVENTION

[0026] In accordance with the press forming process of the presentinvention, the pad portion is prepared to absorb deformation which willoccur, by a material added for producing the pad portion being stretchedevenly to parts of the curved portion which is otherwise would becompleted under high tension, so that, in the present invention, springback caused by such high tension can be minimized. A primary die usedfor the present invention has a simple shape only with the pad portion.Therefore, it has been possible to obtain a product by press formingusing a die which is roughly in the same shape as that of the workpiece.This makes it easy to determine the shape of the die, thereby saving thecost and time for manufacturing the die, and shortening the operatingtime.

[0027] Other feature of this invention will become understood in thecourse of the following description of embodiments, which are given forillustration of the invention and are not intended to be limitingthereof.

[0028] In the present invention, a frame 1 as shown in the perspectiveview of FIG. 1 is obtained by a press forming process. The frame extendswith a curved portion for instance as indicated by a reference numeral4, having an approximate U-shaped cross-section formed with a top face 2and lateral walls 3 provided at both sides of the top face 2. The pressforming process for obtaining such a product will now be explained.

[0029] The process of the invention was attained for minimizing thespring back at the curved portion 4, which has occurred in the productproduced by known methods. Such a spring back phenomenon occurs afterthe product with the curved portion under tension is prepared. Thetensioned part of the product cannot maintain its shape and tends toinappropriately bend.

[0030] The product 1 is produced by a die composed of a female die part11 and a male die part 12 with plate holding members 10 as shown in FIG.2. Because of the aforementioned reason, a short length at the curvedportion 4 tends to appear, with respect to the corresponding length ofthe plate holding members 10. Therefore, a workpiece of the presentinvention has a primary curved portion with a pad portion as will bedescribed in detail below, which is for preparing a product 1 includinga curve part with enough length to minimize the occurrence of the springback.

[0031] In an embodiment of the invention, drawing is carried out toobtain a primary formed article (i.e. a workpiece) with a pad portion 6for the compensation of the short length to be appeared on a finalproduct. The pad portion 6 is obtained as illustrated in FIG. 3 bydrawing by a primary die for the primary formed article 1 a, which padportion 6 has a plurality of concave portions 6 a with spacetherebetween, each extends almost vertically at the primary curvedportion 4 a crossing with the running direction of the primary formedarticle 1 a. By the provision of a plurality of concave portions 6 a,the primary curved portion 4 a has a cross-section in a wave shape asshown in the partially broken view. The product 1 to be obtained afterthe secondary press forming is shown by an alternate long and two shortdashes line.

[0032] Subsequently, the thus formed primary formed article 1 a issubjected to press forming by a secondary die having a shape that issubstantially the same as that of a product to be obtained. In the pressforming, the primary curved portion 4 a is subjected to a stretchingpress of the workpiece.

[0033] The pad portion 6 is pressed out in the direction shown by anarrow F while maintaining a continuous shape to a primary formed lateralwall 3 a, whereby the primary curved portion 4 a can be smoothlystretched, that is, without any irregularity, along the face of theabove-mentioned plate holding member employed with the secondary die. Asa result, the internal stress caused in the curved portion 4 isminimized, whereby the deformation of the obtained formed product byspring back can be satisfactorily eliminated. Finally, a frame 1 asshown in FIG. 1 is obtained with high dimensional accuracy.

[0034]FIG. 4 shows a cross-section of the frame 1 as a product shown inFIG. 1 cut along I-I for comparing the shape with respect to across-section of the primarily formed article 1 a shown in FIG. 3.

[0035] Furthermore, FIG. 5 is a cross-section obtained by cutting thecross-section of FIG. 4 along II-II and seen from the top. The primaryformed article 1 a of which primary formed lateral wall 3 a should beinwardly configured from the lateral wall 3 of the product 1 as shown inFIGS. 4 and 5. Since the primary formed article 1 a in the presentinvention is prepared without having part protruded outwardly beyond theproduct 1 or the die, it is possible to avoid interference between theprimary formed article 1 a and the secondary die when the press formingis performed. Thus, the press forming is smoothly carried out.

[0036] As described above, the primary formed article 1 a is obtained ina simple shape having a plurality of concave portions 6 a at the primarycurved portion 4 a by the die having a shape that is very close to thatof the product. According to the present invention, it is possible toeasily determine the shape of the die to be used since no deformationsubstantially occurs on the final product. Therefore, the cost formanufacturing the die is saved, the time period for manufacturing thedie is shortened, and in addition, the forming operating time ofminimized.

[0037] Furthermore, it is preferable that the edge length and shape ofthe primary curved portion be made to be approximately the same as thatof the face of the plate holding member. The deformation caused by thespring back can be completely eliminated by making the primary curvedportion correspond to the face of the plate holding member.

[0038] In the above embodiment, it is also possible to prepare the padportion 6 in a bead or corrugation shape or the like instead of theabove-mentioned wave shape.

[0039] The present invention has been explained based on the embodimentfor manufacturing a frame for a vehicle by press forming. In thealternative, the method of the invention can be applied for themanufacture of any other products obtained by press forming wherein astretching press operation is carried out.

[0040] As is understood from the above, the present invention provides apress forming process by which the product with high dimensionalaccuracy can easily be obtained, by the preparation of the pad portionat the part where a stretching press is carried out. By the process ofthe invention, the deformation of the product by spring back can beeliminated. In the present invention, it is not necessary to prepare adie which has a shape that is largely different from the product to beobtained. Additionally, it is possible to press the primary formedarticle by the die which has a similar shape to that of the product. Theshape of the die employed for the press forming can be readilydetermined under the pre-assumption of a deformation behavior of theworkpiece. Hence, it is possible not only to save the manufacturing costof the dies and the preparation time period thereof, but also to shortenthe operating time.

[0041] The invention being thus described, it will be understood thatthe same may be varied in many ways. Such variations are not to beregarded as a departure from the spirit and scope of the invention, andall such modification as would be understood to one skilled in the artare intended to be included within the scope of the following claims.

What is claimed is:
 1. A press forming process using a stretching pressto form a product with a curved portion from a workpiece, comprising:providing a pad portion to said workpiece prior to starting the pressforming process, completing a formation of said workpiece by spreadingsaid pad portion mainly in a longitudinal direction of said workpiece,whereby said curved portion is prepared so as to have a spring back ofsaid product minimized.
 2. The press forming process as claimed in claim1 , wherein said pad portion is at least one of a wave, a bead and acorrugation form.
 3. The press forming process as claimed in claim 1 ,wherein said workpiece is made of a steel with a high tensile strength.4. The press forming process as claimed in claim 2 , wherein saidworkpiece is made of a steel with a high tensile strength.
 5. The pressforming process as claimed in claim 1 , wherein said pad portion has ashape avoiding an interference with a die to be used for said stretchingpress.
 6. The press forming process as claimed in claim 2 , wherein saidpad portion has a shape avoiding an interference with a die to be usedfor said stretching press.
 7. The press forming process as claimed inclaim 3 , wherein said pad portion has a shape avoiding an interferencewith a die to be used for said stretching press.
 8. The press formingprocess as claimed in claim 1 , wherein the edge length of a portion ofthe workpiece to be stretched is substantially the same as that of theface of a plate holding member of a die used for the stretching press 9.The press forming process as claimed in claim 2 , wherein the edgelength of a portion of the workpiece to be stretched is substantiallythe same as that of the face of a plate holding member of a die used forthe stretching press.
 10. The press forming process as claimed in claim3 , wherein the edge length of a portion of the workpiece to bestretched is substantially the same as that of the face of a plateholding member of a die used for the stretching press.
 11. The pressforming process as claimed in claim 5 , wherein the edge length of aportion of the workpiece to be stretched is substantially the same asthat of the face of a plate holding member of a die used for thestretching press.
 12. A product pre pared by a press forming processusing a stretching press to form a product with a curved portion from aworkpiece, said press forming process comprising: providing a padportion to said workpiece prior to starting the press forming process,and completing formation of said curved portion by spreading of said padportion mainly in the longitudinal direction of said workpiece, wherebysaid curved portion is prepared so as to have a spring back of saidproduct minimized.
 13. A workpiece comprising a portion including a padportion for providing a dimensionally accurate product used in a pressforming process.